Layer-2 to mpls service mediation architecture

ABSTRACT

An architecture for providing service mediation in a network having a Layer-2 domain and an MPLS domain includes at least one Layer-2 provider edge device in communication with a first customer site; at least one Layer-2 edge device in communication with the Layer-2 provider edge device; at least one MPLS mediation edge device in communication with the Layer-2 edge device; and at least one MPLS provider edge device in communication with both the MPLS mediation edge device and a second customer site. An end-to-end connection is established using native Layer-2 signaling, if any, in the Layer-2 domain and PWE3 signaling protocols in the MPLS domain. The MPLS mediation edge device resolves associations between Layer-2 edge devices and MPLS provider edge devices. The service is “mediated” in the sense that native Layer-2 signaling is terminated at the MME, and a new domain, i.e., pseudowire, is established across the MPLS domain.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/167,883, filed Jun. 27, 2005, entitled LAYER-2 TO MPLS SERVICEMEDIATION ARCHITECTURE, which claims priority to U.S. ProvisionalApplication Ser. No. 60/583,381, filed Jun. 28, 2004, entitled LAYER-2TO MPLS SERVICE MEDIATION ARCHITECTURE, the entirety of both which bothare incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to the field of network communications,and more particularly to establishing dynamic end-to-end mediatedconnections from a Layer-2 network to an MPLS network.

BACKGROUND OF THE INVENTION

The following documents are incorporated by reference:

-   [SPVC-IW] Swallow draft “Soft Permanent Virtual Circuit Interworking    between PWE3 and ATM”, draft-swallow-pwe3-spvc-iw-00.txt-   [PWE3-CONTROL] Martini, L., et al., “Pseudowire Setup and    Maintenance using LDP”, draft-ietf-pwe3-control-protocol-05.txt,    December 2003-   [L2VPN-SIG] Rosen, E., Radoaca, V., “Provisioning Models and    Endpoint Identifiers in L2VPN Signaling”,    draft-ietf-12vpn-signaling-00.txt-   [VPN-AUTO-DISCOVERY] Ould-Brahim, H., Rosen, E., Rekhter, Y., “Using    BGP as an Auto-Discovery Mechanism for Provider-Provisioned VPNs”,    draft-ietf-13vpn-bgpvpn-auto-00.txt.-   [BGP-L2VPN-AD] Radoaca, Unbehagen, P., et al., “BGP-based    Auto-Discovery for L2VPNs”, work in progress,    draft-hlmu-12vpn-bgp-discovery-00.txt-   [SM-SCOPE-REQ] “Service Mediation scope and requirements”,    mpls2004.010.00-   [SHAH-QOS] Shah, H., Ould-Brahim, H., Metz, C., “QoS Signaling for    PWE3”, draft-shah-pwe3-pw-qos-signaling-00.txt, work in progress.

Layer-2 protocols such as Frame Relay and Asynchronous Transmit Mode(“ATM”) were developed with the intent of supporting voice and datacommunications in service provider networks. Consequently, Layer-2legacy equipment is widely deployed in existing networks. More recently,service providers have been deploying Internet Protocol (“IP”) andMulti-Protocol Label Switching (“MPLS”) based equipment. Since it wouldbe quite costly to deploy parallel networks, service providers oftendeploy IP and MPLS based equipment in the same network with legacyequipment based on Layer-2 protocols.

One provisioning model for a network that includes both ATM/FR equipmentand IP/MPLS equipment is the unmapped mode. The unmapped mode requiresthat calls originating from the Layer-2 domain encode information thatidentifies the target Forwarder and the destination MPE in the MPLSdomain. The Forwarder located on the MPE side is identified usingLayer-2 related information such as port number, VPI, VCI, and DLCIvalues. Forwarder identifiers at the MPE and the IP address of the MPEare taken exclusively from the Layer-2 information carried within thenative Layer-2 call. The attachment identifiers on the MPE representinformation relevant to the Layer-2 network being mediated. The nativeLayer-2 address represented by the Calling Party Number (in this casethe ATM NSAP address) encodes the IP address of the destination MPE andis assigned a specific address format code that indicates that theaddress contains an IP address. During the signaling phase the mediationfunction screens the Called Party Information Elements and extract theIP address, the port number and the VPI.VCI values. The MME screens theAFI and ICP from the received call, extracts the IP address representingthe loopback address of the destination MPE, and establishes apseudowire to that MPE. A TAII is constructed from the informationcarried within the NSAP address (in this case the ESI—End SystemIdentifier) and the SPVC IE (VPI.VCI/DLCI values). An Example of theunmapped mode solution is described in [SPVC-IW], which is limited toATM technology. However, the unmapped mode has some disadvantages.

The unmapped mode disadvantageously restricts the set of Forwarders onthe MPE devices to only those that are relevant to the Layer-2 network.Indeed, it is not possible for the MPLS operator to configure twoseparate Forwarders for the same mediated service on two different MPEswith the same or different VPI.VCI/DLCI values or port numbers. Neitheris it possible for the unmapped mode to support scenarios where anEthernet port is backing up a primary ATM port on the MPE, or an ATMport is upgraded to an Ethernet port without reconfiguring all theLayer-2 connections on the Layer-2 network domain destined to that MPE.Further, it is not clear how the unmapped mode supports other addressingplans such as E.164 and X.121 which can still be found in some FrameRelay networks today (since this mode requires encoding an IP address ofthe MPE onto the native Layer-2 address information). Finally, theunmapped mode does not offer the ability to perform address mobilitywithin the MPLS network such as relocating the attachment identifiers todifferent ports, or to different MPEs, or even to different MPLSnetworks without requiring modifications to these identifiers.

SUMMARY OF THE INVENTION

In accordance with the invention a mapped mode is employed to establishdynamic end-to-end mediated connections from a Layer-2 domain to anIP/MPLS domain. In one embodiment apparatus operable to provide servicemediation in a network having a Layer-2 domain and an MPLS domainincludes: in the Layer-2 domain, at least one Layer-2 provider edgedevice in communication with a first customer site; and at least oneLayer-2 edge device in communication with the Layer-2 provider edgedevice; and in the MPLS domain, at least one MPLS mediation edge devicein communication with the Layer-2 edge device; and at least one MPLSprovider edge device in communication with both the MPLS mediation edgedevice and a second customer site, the MPLS mediation edge deviceincluding logic operable to resolve associations between one of theLayer-2 edge devices and one of the MPLS provider edge devices.

In another embodiment of the invention a method for providing servicemediation in a network having a Layer-2 domain and a Multi-ProtocolLabel Switching (“MPLS”) domain, includes the steps of: receiving aprompt to setup a connection; and resolving an association between aLayer-2 edge device disposed in the Layer-2 domain and a MPLS provideredge device disposed in the MPLS domain, thereby identifying aconnection that traverses both the Layer-2 domain and the MPLS domain.

One advantage of the mapped mode is the flexibility it provides inidentifying the Forwarder on the MPLS network. The Forwarder identifierson the MPE can encode a Layer-2 address that is routable within theLayer-2 network. With respect to the MPLS network, the Layer-2 addressis treated as a bit string. In particular, the MPLS network need nothave any knowledge that the bit string contains a Layer-2 address, northat the MPLS network is required to know what type of address is beingused. Further, no modifications are required to existingpseudowire/L2VPN signaling mechanisms.

Another advantage of the mapped mode is that it does not require theL2PE and the native Layer-2 address to encode and to know a priori theIP address of the MPE. Thus, this mode can support addressing plansother than ATM NSAP and provides address mobility on the MPE side. Italso follows that the mapped mode can take advantage of anauto-discovery mechanism where IP addresses of MPEs are discovered byMMEs along with their associated set of attachment identifiers. Anotheradvantage of this model is that it offers benefits of using GeneralizedID FEC as described in [L2VPN-SIG], including service mobility, serviceresiliency with different AII, and offloads the operator from managing,on the MPLS network, Forwarder identifiers that are relevant only to theLayer-2 network.

In an alternative embodiment an auto-discovery mechanism is employed tofacilitate the mapped mode. Auto-discovery enables the Provider tosupport service endpoint mobility of the Layer-2 endpoints residing onthe MPLS core network. Further, auto-discovery enables the MME todynamically learn the set of remote endpoints with their MPE IPaddresses. Auto-discovery also enables the L2PE to use any native layer2 addressing plan without requiring synchronization between the MPLScore network management and Layer-2 network operations in terms ofaddress management of the MPE devices (i.e., any change to MPEaddressing, etc does not require configuration changes to the layer 2network connections). Further, for calls originating from the MPEs anddestined to the Layer-2 networks, the use of an auto-discovery mechanismallows the MPEs to discover the set of MME addresses and the set of AGIssupported within these MMEs.

BRIEF DESCRIPTION OF THE DRAWINGS

A FIG. 1 is a network reference diagram for Layer-2 to MPLS ServiceMediation.

FIG. 2 illustrates the discovery phase of the mapped mode.

FIG. 3 illustrates the signaling phase of the mapped mode.

FIGS. 4 and 5 illustrate service resiliency.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a network reference diagram for Layer-2 to MPLSService

Mediation. The network includes at least one native Layer-2 domain (100)which is in communication with an MPLS domain (102). The native Layer-2domain employs Frame Relay or ATM, and includes a Layer-2 Provider Edge(“L2PE”) device (104) and Layer-2 Edge (“L2E”) devices (106). The MPLSdomain includes MPLS Mediation Edge (“MME”) devices (108) and an MPLSProvider Edge (“MPE”) device (110). The MPE (110) interconnects customersites that are attached to the MPLS domain (102). The L2Es (106) andMMEs (108) interconnect the Layer-2 domain with the MPLS domain. The MPEand MMEs may, in practice, be the same type of device. The L2PE (104) isa Layer-2 switch which interconnects customer sites that are attached tothe Layer-2 domain. The L2Es (106) are Layer-2 devices that interconnectto the MMEs (108) via a Mediation Interface (“MI”) (112). The MI couldbe based on, for example, FRF.10/X.76, PNNI, and AINI.

In accordance with the invention, an end-to-end connection isestablished using native Layer-2 signaling, if any, in the Layer-2domain (100), and PWE3 signaling protocols in the MPLS domain (102). Theservice is “mediated” in the sense that native Layer-2 signaling isterminated at the MME (108), which performs a Mediation Function, and anew domain, i.e., pseudowire, is established across the MPLS domain.Setup of a mediated service requires associating a Forwarder at oneendpoint, e.g., L2PE (104), with a Forwarder at the correspondingendpoint, e.g., MPE (110). Since a mediated service is constructed fromtwo domains, the end to end mediated connection can be identified by thefollowing tuple:

{<CIP,CDP>, <<MME IP Address, AGI, AII at MME>,<MPE IP Address, AGI, Allat MPE>>};

where:

“CIP” is the Calling End Party;

“CDP” is the Called End Party;

“AGI” is the Attachment Group Identifier; and

“AII” is the Attachment Individual Identifier,

and where with respect to the Layer-2 network, the Layer-2 connection is

established between CIP and CDP Forwarders.

Referring now to FIG. 2, in a mapped mode of service provisioning theLayer-2 calls carry information that maps to one or more targetForwarder identifiers using their associated MPE addresses. Theidentifiers need not necessarily have a relationship to the Layer-2network, which may be advantageous because it may be desirable to setupa mediated service between two Forwarders, one on the Layer-2 networkand the other on the MPLS network, that have no relationship betweenthem in terms of Forwarder identification and do not have knowledge onwhat device these identifiers are located. For example, an operator mayprovision an attachment identifier bit string (200) on the MPE side suchas “Sally's Bakery,” and on the L2PE a Calling Party (CIP) (202)containing the Layer-2 address of “123456789,” with VPI.VCI values of100.20. In order for the “123456789/100.20” Forwarder, i.e., L2PE (104),to establish a mediated connection to the “Sally's Bakery” Forwarder,i.e., MPE (110), the MME (108) must be able to map the CIP/CDP to the“Sally's Bakery” Forwarder. The MME also needs to know on which MPE“Sally's Bakery” is located.

The mapped mode employs a mapping function at the MME (108) to resolvethe association between CIP/CDP and <SAII/TAII, MPE-IP address> values,and vice versa. This mapping can be provisioned at the MME ordynamically discovered during either an auto-discovery phase or asignaling phase, thereby producing an MME discovery table (204). Themapping function can be based on the Calling Party Information or theCalled Party information or both. In the case of a mapping based oncalled party information, and for attachment identifiers on the MPE thatare non Layer-2 address based, i.e., do not encode information such asNSAP addresses, the Called Party number must correspond one-to-one tothe Target Attachment Identifier (“TAI”). Using the called partyinformation the operator will provide, for each Forwarder identifier onthe MPE, a corresponding Layer-2 address and its associated SPVCinformation—VPI.VCI, DLCI values. A potentially more scalable approachis for the MME to use the Calling Party Information, i.e., Calling PartyNumber and SPVC IE, to guide the mapping function to which TargetAttachment Individual Identifier (“TAII”) and MPE IP address to use (forcalls originating from the Layer-2 network), rather than using theCalled Party Information. In this more scalable approach the CalledParty information (“CDP”) is only required to represent informationabout reaching the MPLS network through a given set of MMEs. Forexample, if the TAII is “Sally's Bakery,” and the Calling PartyInformation is 123456789/100.20, and the MME is in the “North America”MPLS region, then calls originating from 123456789/100.20 will be sentto a called party that represents the “North America” region. When theLayer-2 calls reach the MMEs then the mapping function to “Sally'sBakery” requires only the calling party information “123456789/100.20.”

If a BGP-based auto-discovery mechanism is used for mapping, the MPE(110) is configured with the Source Attachment Individual Identifier(“SAII”) and TAII, which are distributed by BGP to the set of MMEs(108). In the illustrated embodiment the SAII will indicate “Sally'sBakery” (the actual forwarder identifier on the MPE) and the TAII is abit string identifier containing the actual Calling Party Information(CIP). In this case it encodes the NSAP address and the VPI.VCI, i.e.,123456789100.20, configured on the L2PE. This approach keeps therelevancy of Layer-2 related information such as called party numberincluding the VPI.VCI, etc only to the Layer-2 network, and it decouplesthe actual called party information from the MPE forwarder information.

In order for the Layer-2 signaling to reach the set of MMEs (108), theMPLS domain needs to be reachable from the Layer-2 domain. This may beaccomplished by provisioning Layer-2 prefix addresses which are routablewithin the Layer-2 domain. For ATM, NSAP summary addresses areconfigured at the edge device (L2E (106) in the case of AINI, MME (108)in the case of PNNI). The Layer-2 domain, and particularly the L2PE, hasa view of which MME node it can reach. The prefix address couldrepresent the entire MPLS domain, or distinct network regions, orspecific nodes, i.e., MPEs.

In the case of Frame Relay, apart from use of FRF.8, destination nodes,distinct network regions, or even the entire MPLS network can beassigned standard Layer-2 addresses of X.121 or E.164 addressing plans.The prefix address can relate to the FRF.10 interface point when suchinterface is used. One possible mechanism for routing Frame Relay callsto the MMEs within the Frame Relay domain is to use hunt group servers.Hunt group servers can be assigned the appropriate prefix address forthe MPLS region to which they will direct the calls. Consequently, whena frame relay call is initiated in the Layer-2 network that call isrouted to the hunt group servers which will pass it along to one of theFRF.10 interfaces (MME).

In the case of calls originating from the MPLS domain (102), the set ofMMEs need to be reachable from the MPLS domain, particularly from theset of MPEs (110). The MPEs can be configured, or auto-discovered, witha list of potential MME IP addresses to use. An MPE that intends toplace a call, e.g., an LSP through sending the Label Mapping, willselect a particular MME given a local policy.

One framework for BGP-based auto-discovery is described in[VPN-AUTO-DISCOVERY]. Detailed BGP-based auto-discovery procedures forL2VPNs are documented in [BGP-L2VPN-AD]. The auto-discovery proceeds byhaving each MPE distributing its set of <SAII,TAII> tuples with itselfas the BGP next hop and with a set of export route target values. Inmost common scenarios the MMEs and MPEs will be clients of a set of BGProute reflectors which will distribute L2VPN information to them. TheMMEs are configured with an import policy that imports NLRIs containingattachment identifiers that are intended to be mediated. Once theinformation is received, each MME will record the actual IP address ofthe remote MPE and its related set of attachment circuits.

FIG. 3 illustrates an example of signaling for mediation between twoframe relay endpoints (300, 302) across the MPLS domain (102). An L2PE(104) initiates establishment of a mediated Layer-2 connection SETUP(304) using native Layer-2 signaling protocols. The SETUP is provided tothe remote attachment circuit or remote MPE (110). It should be notedthat no special knowledge is required at the L2PE regarding the mediatednature of the service. Further, the MME should be in ready state able toreceive incoming calls from the Layer-2 network and will have thefollowing information already available:

1) The list of forwarder identifiers. In this case the MME will build atable (204 FIG. 2) of <SAII, TAII, MPE IP addresses>. The SAII is theforwarder identifier on the MPE side. The TAII point to Calling PartyInformation elements, i.e., combination of Calling Party Number and SPVCIE; and

2) The list of IP addresses of the destination MPEs corresponding to thelist of <SAII,TAII>.

For the Frame Relay Layer-2 network the calling and called addresses areof either the E.164 or X.121 addressing plans (assuming FRF.8 is notused). In the case of an ATM service, the calling and called addressesare NSAP based addresses. The call is then routed to the L2E (106) andsubsequently to the MME (108) which will perform the followingfunctions:

-   -   1. The MME will screen the Layer-2 information carried within        the SETUP message and perform normal Layer-2 functions (e.g.,        CAC, QoS, etc) at the MI (112). If these functions fail then the        call is cleared. If not then the following steps are performed.    -   2. For ATM the MME will screen the Called Party Number and        particularly the AFI and ICP values. If the AFI and ICP values        indicate values of “35” and “0002” then the called party number        encodes the IP address of the destination MPE. The mediation        function will then operate using the unmapped mode. The MME        extracts the ESI value and combines it with information from the        SPVC IE to construct the TAII. Detailed procedures for this mode        are described in [SPVC-IW].    -   3. For calls that do not encode an IP address in their Called        Party Number, the MME will screen the Calling Party Information        Elements (Calling Party Number and SPVC IE). That information is        then compared to the list of AIIs learned (or configured) from        the set of MPEs.    -   4. If a match is found then the MME will extract the AII (which        is the actual forwarder identifier on the MPE side) and its        corresponding IP address of the destination MPE. This AII        becomes the TAII with respect to the MME.    -   5. The MME establishes a targeted LDP session to the remote MPE        if one does not already exist.    -   6. The MME will format the Generalized ID FEC TLV (GID) where        the TAII field is the forwarder identifier of the attachment        circuit on the MPE, and the SAII is built either from the actual        source address of the call, or is taken from the list of        available AII residing on the MME. Once the GID is constructed        the MME will send its label mapping to the MPE indicating its        intention to establish a pseudowire to the TAIL    -   7. Upon receiving the Label Mapping, the MPE follows the        procedures described in [PWE3-CONTROL] and if the Label Mapping        is accepted it initiates a reverse label mapping to the MME.    -   8. Once the label mapping is received from the MPE, the MME will        format a native Layer-2 connect/accept message destined to the        remote L2PE and the end to end Layer-2 connection is        established.

It is also possible to establish a mediated service where the MPLSdomain, particularly the MPE, initiates the signaling procedures insteadof the Layer-2 domain. In this mode of operations each MPE is providedwith a list of MMEs to be used for mediation purposes. This list can beconfigured or auto-discovered. Once a particular MME is selected, theMPE establishes a targeted LDP session to that MME and initiates thesignaling procedures as follows:

-   -   1. The MPE builds the Generalized ID FEC with a TAII that        encodes the actual Called Party Number. The Called Party Number        represents the address in the Layer-2 network of the L2PE        endpoint.    -   2. The MPE provides additional information that can be used to        construct the called SPVC IE such as VPI.VCI, DLCI numbers, etc.        This information can be encoded in a separate TLV.    -   3. The MPE provides sufficient information to the MME about QoS        related information of the mediated service. One example of QoS        procedures that can be used are described in [SHAH-QOS].    -   4. The SAII encodes the actual forwarder identifier of the MPE        attachment circuit. That identifier can be any bit string value.        The AGI value is the same as in the Layer-2 initiated approach        and represents an identifier for the Layer-2 region. Once the        Label Mapping is received by the MME, the following procedures        are performed:    -   5. The MME constructs the Called Party Information Elements        (CDP) from the TAII value and the SPVC IE from the Service        Mediation TLVs and the traffic management descriptor from the        QoS TLV.    -   6. The Calling Party Information Elements (CIP) may contain the        Layer-2 prefix address or another address that is assigned for        that particular MME. The calling VPI.VCI/DLCI values will        indicate an attachment circuit selected for this connection at        the MME device.    -   7. The call is then sent to the destination L2PE which will        process the call using normal Layer-2 procedures.    -   8. If the call is accepted then the L2PE sends its connect or        accept message to the MME.    -   9. The MME will then initiate the reverse label mapping destined        to the remote MPE and the end-t0-end mediated connection is        established.

FIG. 4 illustrates potential points of failure of the mediationarchitecture. Resiliency techniques may be employed in response tofailures. Resiliency in the Layer-2 domain may also be employed inresponse to failures, but is a function of the Layer-2 network and istherefore well known in the art. In case of failure of the MI, L2E, orMME failures depicted as (400, 402 and 404, respectively), the call iscleared and a new call is rerouted to an alternate MME (410), which willprocess the call according to steps described in the signaling section.In the case of ATM, automatic rerouting of connections upon networkfailures can be achieved by PNNI with the primary and secondary MMEconfigured with the same NSAP summary address. As long as a suitablepath exists between the calling PNNI node and the alternate MME node,PNNI can recover from any link or node failure in order to complete thecall to the MME. In the case of AINI, a similar rerouting mechanism canbe used. If the AINI link fails then the summary address provisionedagainst the AINI link is withdrawn from the PNNI network. Alternate MMEscan be provisioned with summary addresses to provide redundancycapabilities. In the case of Frame Relay, the Layer-2 network will haveto select an alternate FRF.10 interface. If hunt group servers are usedwithin the Layer-2 network and in the scenario that a particular FRF.10interface (to the MME) becomes unavailable, the hunt group server willbe notified of the failure and a new call will be routed to an alternateMME.

For calls originating from the MPLS network, a failure (404) of the MMEwill cause a global repair of the mediated connection. In this case, theinitial Layer-2 call is cleared and the service label is released. TheMPE (110) will then select an alternate MME (410) based on some internalpolicy such as the proximity (select the closest MME), or availability(selects the one that is available), or in rotary mode, or based on somepredefined MME preferences.

A failure beyond the MME such as failure points (406) and (408) maycause the pseudowire to be released and therefore the mediated serviceto be cleared. It may happen however that the MPLS domain will havedeployed recovery mechanisms such as FRR. In this case, failures of theMME-MPE tunnels can be locally repaired without affecting the end to endmediated service.

FIG. 5 illustrates a primary link or MPE failure (408). In this case,the mediated service is configured with a primary attachment circuit(500) and a backup attachment circuit (502) on same or different MPE.The primary and backup attachment circuit identifiers are unique withinthe AGI. The MME will build a backup table (504) of <primary AII, BackupAII> and information regarding the IP addresses of the primary andbackup MPEs. This table can be populated by employing the auto-discoverymechanism or can be manually configured. It should be noted that thereis no restriction on the type of backup identifier to be used or theservice type of the backup. An ATM VC port can be backed up by anEthernet VLAN or raw Ethernet port. Also note that a given primary maybe associated with multiple backup attachment circuits.

Selection of a backup attachment circuit by the MME can be a localpolicy at the MME, or can be guided by information advertised by eachbackup MPE. For example each backup attachment circuit might indicate apreference level. Upon failure of the primary attachment circuit (500)(or the primary MPE) the MME may decide to clear the Layer-2 call andsubsequent calls will be directed to the backup attachment circuit(502). Another option is for the MME to perform a local repair andestablish a pseudowire to the backup attachment circuit withoutimpacting the Layer-2 connection established on the Layer-2 network.

While the invention is described through the above exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modification to and variation of the illustrated embodiments may bemade without departing from the inventive concepts herein disclosed.Moreover, while the preferred embodiments are described in connectionwith various illustrative structures, one skilled in the art willrecognize that the system may be embodied using a variety of specificstructures. Accordingly, the invention should not be viewed as limitedexcept by the scope and spirit of the appended claims.

1. A method of providing service mediation in a network having a firstdomain and a second domain, the method comprising: receiving signalingrequesting that a connection be set up; and resolving an associationbetween an edge device of the first domain and an edge device of thesecond domain without reference to an address encoded by the edge deviceof the first domain to thereby identify a destination in the seconddomain for the connection that traverses both the first domain and thesecond domain.
 2. The method of claim 1, wherein resolving anassociation between an edge device of the first domain and an edgedevice of the second domain comprises using a VPN auto-discoverymechanism.
 3. The method of claim 1, wherein the first domain is aLayer-2 domain and the second domain is a Multi-Protocol Layer Switching(MPLS) domain.
 4. The method of claim 3, wherein the first domain is oneof a frame relay domain, an Asynchronous Transfer Mode (ATM) domain andan Ethernet domain.
 5. The method of claim 3, wherein resolving anassociation between an edge device of the first domain and an edgedevice of the second domain comprises resolving at least one associationbetween a Calling Party/Called Party (CIP/CDP) and a Source AttachmentIndividual Identifier/Target Attachment Individual Identifier MPLSprovider edge address (SAII/TAII, MPE-IP address).
 6. The method ofclaim 1, wherein resolving an association between an edge device of thefirst domain and an edge device of the second domain comprises reading apre-configured table of associations.
 7. The method of claim 6, whereinthe pre-configured table of associations comprises at least one mappingbased on calling party information.
 8. The method of claim 6, whereinthe pre-configured table of associations comprises at least one mappingbased on called party information.
 9. The method of claim 1, furthercomprising configuring a primary attachment circuit and a backupattachment circuit.
 10. The method of claim 9, further comprisingdynamically building a backup table comprising primary attachmentidentifiers, backup attachment identifiers and address information forprimary and backup edge devices of the second domain.
 11. The method ofclaim 1, wherein the network comprises: in the first domain: at leastone first domain provider edge device in communication with a firstcustomer site; and at least one other first domain edge device incommunication with the provider edge device; and in the second domain:at least one mediation edge device in communication with the at leastone other first domain edge device; and at least one second domainprovider device in communication with the mediation edge device and witha second customer site.
 12. The method of claim 11, wherein resolving anassociation between an edge device of the first domain and an edgedevice of the second domain comprises resolving an association betweenthe first domain provider edge device and the second domain provideredge device.
 13. The method of claim 12, wherein the mediation edgedevice is configured to resolve the association between the first domainprovider edge device and the second domain provider edge device.
 14. Themethod of claim 13, wherein the first domain is a Layer-2 domain and thesecond domain is a Layer-x domain where “x” is greater than
 2. 15. Themethod of claim 13, wherein the first domain is a Layer-2 domain and thesecond domain is a Multi-Protocol Layer Switching (MPLS) domain.
 16. Themethod of claim 15, wherein the first domain is one of a frame relaydomain and an Asynchronous Transfer Mode (ATM) domain.
 17. The method ofclaim 15, wherein resolving an association between an edge device of thefirst domain and an edge device of the second domain comprises resolvingat least one association between a Calling Party/Called Party (CIP/CDP)and a Source Attachment Individual Identifier/Target AttachmentIndividual Identifier MPLS provider edge address (SAII/TAII, MPE-IPaddress).
 18. The method of claim 11, wherein the mediation edge devicecomprises a pre-configured table of associations.
 19. The method ofclaim 18, wherein the pre-configured table of associations defines atleast one primary attachment circuit and at least one backup attachmentcircuit.